6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



were no petals in these regular ecliinoids, and the whole ambulacrum 

 was used partially for respiration. Most of the adoral portion of the 

 test was not in contact with the sea floor, but was elevated by spines 

 and by the spherical shape of the test. 



With the development of the irregular echinoid, the living habits 

 changed radically. Prior to this change, most echinoids probably 

 lived on the top of the sea floor, but in the Jurassic the irregular 

 echinoids began to burrow shallowly into the substrate. In order to 

 burrow, many changes in the morphology of the echinoid were neces- 

 sary. Many regular echinoids were aided in respiration by large gills 

 situated around the mouth. These gills could not function when the 

 echinoid was partially buried, and their function was transferred 

 adapically with the development of the petals. The tube-feet in the 

 adapical ambulacra altered greatly in shape, with a great lateral ex- 

 pansion of the outer branch of each tube- foot, greatly increasing its 

 oxygen-absorbing area. Furthermore, the test became greatly 

 flattened, with the reduction in size of the spines, its lower surface 

 came in contact with the sea floor. The adoral tube- feet no longer 

 could be used very effectively for respiration, and they were adapted 

 fot food gathering. The phyllodes resulted from the crowding of 

 these tube-feet around the peristome. The tube-feet between the 

 phyllodes and the petals assumed a sensory function, as in the living 

 spatangoids (Nichols, 1959, p. 399). Simultaneously with these de- 

 velopments, the jaws disappeared, and the eating habits of the echi- 

 noids altered greatly. Echinoids with jaws ate larger food particles, 

 but without jaws the particle size was greatly reduced, necessitating 

 ingestion of many more particles. Presumably the great increase in 

 the number of tube-feet around the mouth resulted from the need to 

 have some means of conveying a great number of food particles to 

 the mouth. Probably these tube-feet later became more specialized 

 for this function, and fewer were needed, explaining the decrease 

 in number of pores in the phyllodes of many of the Cretaceous cas- 

 siduloids. According to Hyman (1955, p. 434), in the living echi- 

 noids the tube-feet of the phyllodes are greatly specialized and are 

 pencillate with the ends of the feet expanded and covered with erect 

 club-shaped projections. Because these tube-feet were not used for 

 respiration, it was not necessary for each foot to have an incurrent 

 and excurrent channel, and for the pores to be paired. Probably the 

 current in a tube-foot not used for respiration is unidirectional at 

 one time, and the partition necessary for a pore pair would obstruct 



